Sun-spot Frequencies Contrasted. 157 



of years there is a decided difference — some 20 per cent. — 

 between the means derived from the 6 winter and the 6 sum- 

 mer months. There is a smaller difference, but in the same 

 direction, between the winter and summer means from the 

 33-year period 1794-1826 remarkable for its low average 

 sun-spot frequency. This is unquestionably somewhat sug- 

 gestive of an appreciable real annual period in sun-spot 

 frequency in years when sun-spots are few; but whereas in 

 the 39 or 33 years of S min. the mean frequency is large for 

 May and November, and small for July, in the period 1794- 

 1826 the exact opposite is seen. Again, an appreciable excess 

 in the winter over the summer mean also appears in the 

 period 1827-59, when the average sun-spot frequency 

 considerably exceeded the average from the whole 153 years. 



§ 7. Proceeding to Table III., we have in the first line 

 the annual variation of auroral frequency in Scandinavia as 

 derived from 117 years. The monthly values represent 

 percentages of the value for the whole year. The largest 

 values occur in October and March, and a secondary 

 minimum is recognizable in December. The dip in the 

 February value arises really from the smaller number of 

 days in that month. When referred to an equal number 

 of days, the February frequency exceeds that in January in 

 the ratio of 104 : 100. 



In the second and third lines we have similarly annual 

 variations from the two 33-year periods of high average 

 sun-spot frequency, their mean appearing in the fourth line. 

 For the ratio, however, between the mean frequencies from 

 the earlier and from the later of these two periods we have 



From Sun-spots. From Auroras. 



100 : ^ 100 : 111. 



If we contrasted these two periods with one another we 

 should thus associate an increase in auroras with a diminution 

 in sun-spots. 



Again, the annual variations from the two periods differ 

 markedly. In the later period, as compared to the earlier, 

 the summer frequencies fall and the winter frequencies rise. 



The differences apparent between the two periods may 

 represent a real change, but in all probability they are largely 

 due to an increase in auroral observers, especially in the 

 northern districts of Scandinavia. Taking the yearly data 

 for the five districts mentioned above, we obtain for the total 

 number of auroras observed the results given in Table IV. 

 (p. 158). 



